1. Field of the Invention
This invention relates to Non-Linear Transmission Line (NLTL) Frequency Multipliers, and more particularly to noise reduction for NLTL Frequency Multipliers.
2. Description of the Related Art
Microwave systems often require high frequency input signals. Frequency multipliers are used to translate a low frequency input signal to a desired higher frequency. Frequency multipliers include a source of the low frequency input signal, a comb generator that produces output signals at multiple harmonics of the input signal, a band pass filter (BPF) that selects one of the harmonics and amplifier.
Conventional comb generators produce the harmonics using step recovery diodes (SRD). SRD implementations generally accept input signals over a narrow range of frequencies and power levels, thereby limiting user selection of harmonics spacing and frequency range. In addition, SRD implementations can introduce substantial phase noise.
A new family of comb generators based on nonlinear transmission line (NLTL) technology has demonstrated improved phase noise and a wider input power range. NLTL generators create output harmonics through the nonlinear nature of propagation within the device, avoiding exposure to recombination and shot noise that is prevalent within step recovery diodes. The aggregate effect of this new technology is that the residual phase noise is dramatically better—NLTL comb generators are exhibiting at least a 20 dB improvement over their SRD counterparts.
Referring now to FIG. 1, a nonlinear transmission line (NLTL) 10 is a transmission line formed from a periodic structure of series inductors 12 and variable shunt capacitors 14. The variable shunt capacitors are suitably voltage sensitive Schottky varactor diodes. The capacitance of a reverse biased Schottky diode is voltage dependent such that the capacitance at low reverse bias is much greater than the capacitance at high reverse bias. An input signal 16 propagating on the equivalent transmission line made with varactors experiences a propagation velocity that is voltage dependent. A signal that transitions from low to high voltage will be compressed in time as the initial low voltage portion of the signal travels down the line slower than the later, higher voltage portion of the signal. Consequently, the higher voltage portion of the waveform “catches up” with the lower voltage portion of the step, resulting in increasing the edge speed of the low to-high transition. This sharper rising edge waveform produces an output signal 18 that is rich in signal harmonics in the frequency spectrum. A more complete description of a NLTL is provided in Mark J. Rodwell et al. “GaAs Nonlinear Transmission Lines for Picosecond Pulse Generation and Millimeter-Wave Sampling” IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 7, July 1991, pp. 1194-1204 and Wenjia Tang et al. “Low Spurious, Broadband Frequency Translator using Left-Handed Nonlinear Transmission Line” IEEE Microwave and Wireless Components letters, Vol. 19. No. 4, April 2009, pp. 221-223, which are hereby incorporated by reference.
Referring now to FIGS. 2 and 3, a frequency multiplier 20 includes a source 22 that supplies an input signal 24 (frequency-domain representation 26) at a frequency Fo, a NLTL 27 that propagates the input signal nonlinearly to produce a sharp rising edge waveform 28 with multiple harmonics 30 of the input signal in the frequency domain, a band pass filter (BPF) 32 that selects one of the harmonics 33 (NF0) as an output signal 34 and an amplifier 35 that amplifies the output signal 34. A more complete description of a frequency multiplier using NLTL technology is provided in U.S. Pat. Nos. 7,462,956 and 7,612,629, which are hereby incorporated by reference.
Source 22 typically includes an oscillator 36 that generates input signal 24 at a given frequency F0. The amplitude level of input signal 24 must match the input range of the NLTL. Typical sources generate the input signal 24 at a fixed amplitude that does not match the NLTL. Typically, the input signal needs to be amplified. In an embodiment, source 22 includes an amplifier 37 that provides a fixed amount of gain, an input attenuator 38 that attenuates input signal 24 so that its amplitude lies in the linear region of amplifier 37 and an output attenuator 40 that attenuates the amplified signal to provide level adjustment to match the input range of the NLTL. Other source configurations are possible.
BPF 32 has a pass band 42 that is approximately centered at the desired harmonic NF0 and sufficiently wide to pass harmonic 33 and side bands 44 that provide sufficient attenuation to reject all other harmonics. Typically, the side bands 44 must satisfy a specified side band rejection requirement 46 (e.g. −40 dB attenuation) at the adjacent harmonics. Filter “Q” determines the width of the pass band 42 and how sharp side bands 44 transition from the pass band level to a high attenuation level. A high Q filter transitions quickly and a low Q filter rolls off slowly. A high Q filter can provide greater side band rejection but is more complex (i.e. a higher order filter), hence costly. Generally speaking, a circuit designer would prefer to select the lowest Q filter that satisfies the side band rejection requirement.